The present invention generally relates to a CRT display apparatus. More specifically, the present invention is directed to a CRT (cathode-ray tube) display apparatus having a high-voltage/precise voltage control apparatus for performing a focusing control and a brightness control (screen voltage control).
In the conventional apparatuses for utilizing high-precision electron beams, such as CRT display apparatuses (involving television receivers), a focusing control is necessarily required by which a high voltage of a focusing electrode is controlled, or adjusted to realize a clear beam spot. Concretely speaking, in a computer color CRT display apparatus with employment of a bipotential tube, a high-voltage focusing control voltage is needed, and a variable voltage dividing circuit (constructed by a potentiometer) for dividing an anode voltage into relatively high voltages of 6 to 8 KV is employed to apply such relatively high voltages to two focusing electrodes in order to follow variations in the anode voltage at high speeds. Then, these voltages are controlled so as to minimize the spot diameters, while a parabolic voltage is applied via a capacitor to one of these focusing electrodes. Similarly, another voltage dividing circuit is used so as to follow the screen voltage. A typical conventional voltage controlling circuit has been opened in, for instance, JP-A-63-203063.
That is to say, this prior art publication describes an improvement in the focusing control of the color CRT display apparatus. The voltage dividing resistor circuit is arranged by such a series circuit constructed of the parallel resistor circuit used to derive convergence voltages for the first and second convergence electrodes with the electrode pieces for constituting the 4-pole lense, and also the resistor circuit provided at the low voltage side. To this convergence-voltage deriving parallel resistor circuit, both of the variable resistor for controlling the first convergence voltage and the variable resistor for controlling the second convergence voltage are connected in a parallel form. These two variable resistors are independently adjusted or controlled, so that preselected optimum DC convergence voltages are applied to the first convergence electrode and the second convergence electrode, and furthermore, the parabolic voltages synchronized with the deflection period are applied to the first and second convergence electrodes. As a consequence, optimum convergence conditions of the electron beams can be obtained over not only the peripheral portion of the screen, but also the central portion thereof, and also the variable screen voltage for controlling the cut-off point can be obtained at the same time.
Since this conventional focusing control operation requires very precise beam spot adjustments, resulting in fatigue of operators, it is preferable to automatically perform such a focusing control operation. However, it is practically difficult to realize an automatic focusing control operation, because the above-described focusing control operation must be carried out under high voltage and precise adjustment.
As to the conventional high voltage/precise controlling method, such an automatic control method may be conceived that a motor-driven screw driver having a precise alignment servo mechanism is used to be fitted to the rotary unit of the above-described focus controlling potentiometer. However, such an automatic focus control method owns the following drawbacks. That is to say, it is rather difficult to maintain reliability over a long time for such a mechanically precise alignment. Moreover, this difficulty is emphasized because the recently developed CRT display apparatuses employ the tilt and swivel mechanism. Accordingly, no automatic control method with high precision and reliability have been developed in view of practical capability.